1. Field of the Invention
The present invention relates to a manufacturing method of a semiconductor device having a countermeasure against static electricity. In particular, the invention relates to a manufacturing method of a semiconductor device using a thin-film semiconductor such as a thin-film transistor formed on a glass substrate.
2. Description of the Related Art
It is known that due to electrification with static electricity (contact electrification, triboelectrification, stripping electrification, or the like) a dielectric material, for instance, is given a very high voltage which amounts to several tens of kilovolts in some cases and a strong electromagnetic field is formed around it. If a conductor such as a metallic material is placed in such an environment, charge may be induced on its surface or polarity separation may occur in the inside thereof, possibly resulting in electrostatic discharge (ESD).
However, it is now pointed out that countermeasures against static electricity in semiconductor factories are insufficient, though the semiconductor industry is growing rapidly. The countermeasures against static electricity means countermeasures for preventing electrostatic breakdown of device elements due to ESD.
The causes of the above situation are the existence of still unclear problems associated with the countermeasures against static electricity and the fact that the countermeasures against static electricity require production of a large-scale facility (environment), an investment therefor would be a heavy economical burden to the management.
For the above reasons, the countermeasures against static electricity tend to be simple ones. In addition, it is said that in many cases effective measures are not taken due to insufficient knowledge of a manager of a manufacturing facility. Examples of commonly employed countermeasures against static electricity are generally classified into a method of grounding an electrified body to thereby quickly leak the charge to the ground and a method of neutralizing the charge of an electrified body with radiated ions.
For example, the method of grounding is effective in a case where an object of charge elimination is electrostatically a conductor (in general, the resistivity is less than 1.0.times.10.sup.6 .OMEGA.m). The method of neutralization with plus or minus ions is effective in a case where an object is moving or is a non-conductor (in general, the resistivity is greater than 1.0.times.10.sup.10 .OMEGA.m).
The reasons for the above facts will be described with reference to FIGS. 3A-3D. FIG. 3A shows a state that a conductor such as a metal is electrified. In this state, the density and the polarity of charge is uniform if there is no influence of an external electric field. In contrast, when a non-conductor such as glass is electrified, both of the density and the polarity is not uniform as shown in FIG. 3B. These phenomena result from high charge mobility of charge in a conductor and absence of charge mobility in a non-conductor.
Further, as shown in FIG. 3C, if an electrified conductor is grounded, it instantaneously loses its charge; that is, charge is eliminated from it. In contrast, as shown in FIG. 3D, even if a non-conductor is grounded, charge elimination takes long time or in some cases substantially no charge elimination is effected because of extremely slow movement of charge.
That is, when a non-conductor is electrified, the charge elimination by grounding is not efficient. Therefore it becomes necessary to neutralize positively electrified regions with minus ions and negatively electrified regions with plus ions.
A common neutralizing method with the use of ions is such that nitrogen molecules, for instance, included in the air are converted into plus and minus ions by changing the amount of electricity thereof by using corona discharge and ions of both polarities are uniformly dispersed in the air. An apparatus for this purpose, which is called an ionizer, is now commonly used.
In the recent semiconductor industry, the mainstream of the development is shifting from thin-film transistors (TFTs) using an amorphous silicon thin film to TFTs using a low-temperature polysilicon thin film. Accordingly, the demand for a technology for forming a semiconductor circuit on an inexpensive glass substrate is increasing further. Needless to say, a glass substrate is insulative; that is, it is a non-conductor.
In a manufacturing process of a semiconductor device, a glass substrate is electrified in various manners, i.e., by contact electrification, triboelectrification, stripping (or separation) electrification, and the like. Charge accumulated in the glass plate forms an electromagnetic field, which in turn induces charge in a conductor such as a wiring formed on the glass plate. If a sufficient amount of charge to cause ESD is induced, resulting discharge may damage device elements.
It is therefore indispensable to eliminate charge from the glass substrate. However, since it is a non-conductor, conventionally there is no effective charge elimination method other than the method of neutralizing charge with ionizer. Actually, in semiconductor device manufacturing companies, ionizers are disposed at proper locations in the entire clean room as a countermeasure against static electricity. Or each substrate is subjected to charge elimination blowing, for instance, to eliminate charge therefrom.
However, in the above methods, it is difficult to control the balance between the amounts of generated ions of both polarities. If a balance is lost, a charge elimination object may unintentionally be electrified with an excessive part of plus or minus ions.
Further, electrode needles of an ionizer for causing corona discharge are lowered in performance due to stain or deformation by galvanic corrosion. Proper maintenance management is therefore needed for the electrode needles. For this reason, at present the use of an ionizer is not a low-cost means as a permanent countermeasure against static electricity. In addition, there is a paper stating that dust that is electrostatically collected on electrode needles may become a source of dust in a clean room.